Nitric Oxide (NO) is generated by a family of isoenzymes, through the enzymatic catalisis of the aminoacid L-arginine, which leads to the production of L-citruline and NO. It is involved in many physiologic processes of mammals, including neurotransmition, blood pressure control, inflammation, immune responses and also in the mechanism of defense against invasive organisms and tumors. The uncontrolled syntesis of NO is thought to take part in the pathogenesis of cardiovascular diseases, auto-immunity, graft rejection, degenerative diseases, as well as in sepsis, genotoxicity and also in neoplasia development. The NO is also considered the initial agent of carcinogenesis, which could lead to uncontrolled cytostasis and cellular differentiation when associated to other factors. The diversity of NO effects seems to be related to the concentration that is generated, to the individual sensibility of cells and the duration of this phenomenon.